This invention relates generally to air conditioning systems and, more particularly, to a method and apparatus for controlling the head pressure of an air cooled condenser.
The condenser of an air conditioning system is normally located out of doors or in heat exchange relation with outdoor air. It is therefore subject to a wide variety of ambient temperature conditions, including very cold conditions. For example, even in the winter, air conditioning is required, and possibly at full capacity, for such applications as cooling computer rooms and the like. During such winter operation, lower outdoor temperatures may materially reduce the condensing temperature of refrigerant in the condenser. For example, a typical air cooled refrigeration system might have a condensing temperature of 125.degree. F. when employing R-22 as refrigerant with an outdoor temperature of 95.degree. F. However, the same system operating at a 0.degree. F. outdoor temperature will cause the condensing temperature to drop as low as 30.degree. F. This, in turn, produces a corresponding reduction in head pressure on the high pressure side of the system. The result is that the pressure at the refrigerant expansion device is substantially reduced such that less refrigerant is passed to the evaporator, and the capacity of the system is therefore reduced. If adequate pressure is not maintained at low ambient temperatures, the evaporator temperature may drop below freezing and cause condensation moisture to freeze on the evaporator coil. The resulting layer of ice tends to insulate the coil from the refrigeration mode and causes a further reduction in system capacity.
A common approach in maintaining an adequate head pressure is to provide a variable speed motor for driving the condenser fan. The condensing temperature is monitored, and when it drops below a predetermined level, the speed of the fan motor is reduced to thereby reduce the capacity of the condenser, thereby raising the condensing temperature. Such an apparatus is described in U.S. Pat. No. 3,196,629 assigned to the assignee of the present invention. While this approach is very affective and has long served the industry, it is recognized that such an approach requires a relatively expensive variable speed motor and control system.
Another approach that has been considered, but which has not been found to be practical for reasons to be discussed hereinafter, is that of cycling a single speed fan motor between equal times of the on and off conditions. Since this brings about a rather rapid response of the system, rapid cycling (e.g. several times per minute) can result. While this may be practicable where several fans are employed, such that the switching can be varied from fan to fan, such a practice has not been practicable heretofore with a single fan since the reliability and life of the necessary switching mechanisms have been inadequate. That is, conventional switching mechanisms would not hold up to the cycling requirement over a prolonged period of time and would therefore be economically unfeasible. Further, it has been found that operation will equal periods of on and off conditions tends to cause the suction pressure to drop to undesirable levels, especially at lower ambient temperatures.
A further complication that results from such a fan cycling approach is that of lubricating the fan motor in a proper manner. For example, as a motor is started up, it is necessary to draw the lubricating oil up into the applicable area surrounding the shaft. During that process, a shaft supported by simple journal bearings would not normally be adequately lubricated and its temperature would tend to increase. Normally this would be acceptable since the supply of the lubricating oil would eventually become adequate and the shaft would then be allowed to cool. However, if the motor is turned off prior to that occurrence and is then soon thereafter turned back on, the supply of lubricating oil would still be inadequate, and the temperature of the shaft would tend to rise further. Eventually, damage would occur to the bearings.
If the frequency of the on-off cycling is such that the motor is on long enough for the oil to reach the bearing, but is then off long enough for the oil to drain out of the bearing, the bearing will again be oil starved upon start-up. Continued operation in this manner may eventually lead to failure. For that reason, in such applications, it has been necessary to use ball bearings in the fan motors. These are considerably more expensive than the sleeve bearings.
It is therefore an object of the present invention to provide an improved method and apparatus for maintaining head pressure in an air cooled condenser of an air conditioning system.
Another object of the present invention is the provision for a head pressure control system which is reliable and of long life.
Yet another object of the present invention is the provision in a head pressure control system for ensuring proper lubrication of the fan motor bearings.
Still another object of the present invention is the provision for a head pressure control system which is effective in use and efficient and economical in operation.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.